Two known types of compressors are the wobble piston type and the swashplate type. The wobble piston type is exemplified by U.S. Pat. No. 3,961,868 issued Jun. 8, 1976, to Droege, Sr., et al. for xe2x80x9cAir Compressorxe2x80x9d. Such a compressor uses a piston whose head has a peripheral seal that seals with a cylinder bore. The piston rod is mounted radially on a crankshaft. The piston includes no joints or swivels. As a result, the piston head is forced to xe2x80x9cwobblexe2x80x9d in two dimensions within the cylinder bore as it is driven by the crankshaft.
The swashplate type compressor uses a plurality of axial cylinders arranged in a circle about a drive shaft. A swashplate is inclined relative to the shaft axis such that the plate gyrates as the drive shaft is rotated. Pistons are mounted in each of the cylinders. The ends of the piston rods are connected to elements that slide over the surface of the swashplate as the swashplate rotates. The result is that the centerline of the piston head is moved solely in an axial direction as the pistons are stroked within the cylinders. An example of such an axial piston swashplate compressor is found in U.S. Pat. No. 5,362,208 issued Nov. 8, 1994 to Inagaki, et al. for xe2x80x9cSwashplate Type Compressorxe2x80x9d. Another example is U.S. Pat. No. 4,776,257 issued Oct. 11, 1988, to Hansen for xe2x80x9cAxial Pump Enginexe2x80x9d. In the Hansen patent, the centerline of the piston heads are inclined relative to the centerline of the cylinder bore, but the piston heads are moved only along the piston head centerline in one direction.
The present invention combines the wobble pistons normally used in radial piston pumps with a nutating plate rather than the swashplate normally used in axial piston pumps. The result is a simple and effective fluid pumping apparatus. A counterweight with particular mass and mass moment of inertia properties provides near perfect balancing of the piston system to reduce vibration and wear.
In accordance with the invention, an axial piston pump has a drive shaft rotatable about a shaft axis. A counterweight is mounted to rotate with the shaft with its axis at an oblique angle to the shaft axis so that its axis precesses about the shaft axis as the shaft rotates. A bearing is mounted on the counterweight and a piston assembly is mounted on the bearing. The piston assembly includes a carrier and at least two wobble pistons mounted to the carrier and spaced apart at equal angles. The piston assembly precesses about the counterweight axis so that the pistons reciprocate along axes parallel to the shaft axis when the shaft rotates. The counterweight produces a moment with respect to the shaft corresponding to the average moment of the piston assembly.
The piston assembly is somewhat self-balanced by virtue of the uniform distribution of the pistons on the carrier. However, some miscellaneous radial and axial forces remain from the moving center of gravity during precession and the effect of non-homogeneous mass concentrations, such as those created by the pistons. Near perfect dynamic balancing is achieved by the counterweight by selecting its moment of inertia and configuring and weighting it to counteract these forces as well as moments that may result from the counteracting forces of the counterweight.
In, particular, the counterweight has a mass component providing a counter balance moment opposing a primary moment about an axis perpendicular to the shaft axis from reciprocation of the pistons and precession of the piston assembly. The counterweight can further include a mass component providing a counter balance force opposing the radial force arising from the piston assembly having a center of gravity spaced from the shaft axis. Still further, the counterweight can have a mass component providing a counter balance moment opposing a moment arising from the aforesaid counter balance force and the center of gravity of the piston assembly being spaced apart axially.
The above mass components can be separate elements mounted to the counterweight. In a preferred form, the counterweight includes these mass components as a monolithic structure. This structure can have a hub defining an eccentric cam surface where the bearing is mounted through which a shaft receiving bore extends. An angled lobe extends toward the piston assembly at an acute angle from the hub. The lobe is eccentric to the hub and extends further from the side of the hub nearest the bore.
Preferably, the pistons are connected to the piston carrier by radially resilient but axially stiff connecting rods. The axial stiffness of the connecting rods is sufficient to exert the required forces of compression and vacuum on the piston without significant change in length of the rod, but is radially resilient so as to reduce the radial loads exerted on the piston seal, and therefore increase the life of the piston seal.
It is a principal object of the invention to provide a simplified axial piston pumping apparatus using wobble pistons with quiet operation, efficient power usage and good longevity without sliding elements requiring continuous lubrication.
It is another object of the invention to provide a highly, near-perfectly, balanced precessing piston assembly.
It is another object to achieve near-perfect balancing of the system with a simple, unitary counterweight component.